The paper introduces a model for *provable data possession* (PDP) that allows a client to verify that data stored on an untrusted server is still intact without retrieving it. The model generates probabilistic proofs of possession by sampling random sets of blocks from the server, significantly reducing I/O costs. The client maintains minimal metadata to verify the proof, and the challenge/response protocol transmits a small, constant amount of data, minimizing network communication. This makes PDP suitable for large datasets in widely-distributed storage systems. The authors present two efficient PDP schemes that are more secure and practical than previous solutions, even those with weaker guarantees. These schemes have low server overhead (constant or linear in the data size) and require minimal client computation. Experiments verify the practicality of PDP, showing that performance is bounded by disk I/O rather than cryptographic computation. The PDP schemes use *homomorphic verifiable tags*, which allow tags for multiple file blocks to be combined into a single value. The client pre-computes tags for each block and stores the file and tags with the server. Later, the client can verify data possession by generating a random challenge against selected blocks, with the server responding with a proof of possession. One scheme, E-PDP, is particularly efficient, using a single modular exponentiation at the server. Another variant offers public verifiability, allowing anyone to challenge the server for data possession. The schemes are designed to be data format-independent and can handle multiple files. The paper includes a detailed security analysis, implementation, and performance evaluation, demonstrating that PDP can effectively detect server misbehavior with high probability, even when a significant fraction of the file is deleted.The paper introduces a model for *provable data possession* (PDP) that allows a client to verify that data stored on an untrusted server is still intact without retrieving it. The model generates probabilistic proofs of possession by sampling random sets of blocks from the server, significantly reducing I/O costs. The client maintains minimal metadata to verify the proof, and the challenge/response protocol transmits a small, constant amount of data, minimizing network communication. This makes PDP suitable for large datasets in widely-distributed storage systems. The authors present two efficient PDP schemes that are more secure and practical than previous solutions, even those with weaker guarantees. These schemes have low server overhead (constant or linear in the data size) and require minimal client computation. Experiments verify the practicality of PDP, showing that performance is bounded by disk I/O rather than cryptographic computation. The PDP schemes use *homomorphic verifiable tags*, which allow tags for multiple file blocks to be combined into a single value. The client pre-computes tags for each block and stores the file and tags with the server. Later, the client can verify data possession by generating a random challenge against selected blocks, with the server responding with a proof of possession. One scheme, E-PDP, is particularly efficient, using a single modular exponentiation at the server. Another variant offers public verifiability, allowing anyone to challenge the server for data possession. The schemes are designed to be data format-independent and can handle multiple files. The paper includes a detailed security analysis, implementation, and performance evaluation, demonstrating that PDP can effectively detect server misbehavior with high probability, even when a significant fraction of the file is deleted.